1973 750 Combat

halcyonhoward · Jun 16, 2021

Greetings all - looking to increase my knowledge regarding my 1973 750 "Low Compression" Combat... Date stamp reads "1972 NOV", VIN 212451...

Has "C" head, 32mm carbs, Combat breather assembly, silver barrel, black gauge holders, steel side covers, stainless fenders, '73 "basket weave" seat, rectangular L917 taillight... Identical in pretty much every way (other than the breather and stamping on head) to my '73 850...

I was always told by "knowledgeable" Norton folks that this bike is a "low compression" Combat, but never got a satisfactory explanation of what this means exactly... How much "lower"? Lower than what?? what was changed relative to earlier Combats (other than the Superblends)?

And - how many of this version of the Combat were produced?

Educate me please!

Thanks in advance from San Jose California!
HJ

L.A.B. · Jun 17, 2021

halcyonhoward said:
"1972 NOV", VIN 212451...

I was always told by "knowledgeable" Norton folks that this bike is a "low compression" Combat, but never got a satisfactory explanation of what this means exactly... How much "lower"? Lower than what?? what was changed relative to earlier Combats (other than the Superblends)?

And - how many of this version of the Combat were produced?

The factory halted Combat production at serial 211110 in order to avoid bankruptcy due to the high number of warranty claims, the situation being so bad that all available unsold Combats were returned to the factory to be 'detuned' after which, the factory never used the "Combat" name again, however, it's possible the Combat head was still available as a special order item because it's not all that unusual to find a post-Combat model with a 'C' head but of course the head could have been fitted sometime after the bike left the factory unless you know its complete history.
No Combat parts (head, camshaft) are listed in the 212278-on parts book (link, below) and the presence of a 'C' head alone (without the camshaft) doesn't make a bike a Combat.

http://www.classicbike.biz/Norton/Parts/1970s/Commando_Parts_73-74.pdf

As it has a 'C' head then logically, it can't be "low compression" as there was only one 'C' head (RH3) and it's the cylinder head that determines compression ratio (assuming standard thickness head and base gaskets), not the pistons.

The post-Combat series 32mm inlet heads were the RH5 (standard or 'low') compression and RH6 ('high' compression but not as high as the Combat head).

Edit:

halcyonhoward said:
Combat breather assembly,

All* 200000+ series 750 models had that breather whether Combat or not (so prior to 200976 when Combat production began, during Combat production and after Combat production ended at 211110) so "Combat breather" (or Combat cases) is misleading.
*(Except the very last 750 models that had '850' cases).

ILLF8ED · Jun 17, 2021

This will help.

David B · Jun 17, 2021

Wow, thanks for posting that chart, David. I hadn't seen that before and didn't realize there were actually that many heads. Very interesting. I presume some are quite rare?

Mart UK · Jun 17, 2021

Is there a way to identify which head you have, without taking it apart?

Flatspot · Jun 17, 2021

My 850 RH10 head is stamped above the right hand exhaust rocker cover, if that helps.

L.A.B. · Jun 17, 2021

Mart UK said:
Is there a way to identify which head you have, without taking it apart?

Do you mean how do you identify the head of your particular Commando? As your Commando is an 850 Mk3 then it is likely to be stamped 'RH4' in the place Landspeedracer described as it seems the factory reverted to the RH4 head for the 850 Mk3.

Ron L · Jun 17, 2021

Interesting. My two 1973 750's both have 11/72 build dates and 22xxxx serial numbers. Both have RH6 heads, standard camshafts, black clock holders, silver barrels, 932 carbs, basketweave seats, and square taillights. Typical of what is considered the 'MkV' 750 (although apparently that designation was never used). Perhaps this was a case of using the stock on hand until it was depleted during the 'change' from MkIV to MkV.

Does your bike have a cylinder base gasket or a cylinder base spacer to lower compression? Have you had the barrels off and checked for the 'SS' stamp (Combat cam)?

L.A.B. · Jun 17, 2021

Ron L said:
Typical of what is considered the 'MkV' 750 (although apparently that designation was never used).

marshg246 · Jun 17, 2021

halcyonhoward said:
Greetings all - looking to increase my knowledge regarding my 1973 750 "Low Compression" Combat... Date stamp reads "1972 NOV", VIN 212451...

Has "C" head, 32mm carbs, Combat breather assembly, silver barrel, black gauge holders, steel side covers, stainless fenders, '73 "basket weave" seat, rectangular L917 taillight... Identical in pretty much every way (other than the breather and stamping on head) to my '73 850...

I was always told by "knowledgeable" Norton folks that this bike is a "low compression" Combat, but never got a satisfactory explanation of what this means exactly... How much "lower"? Lower than what?? what was changed relative to earlier Combats (other than the Superblends)?

And - how many of this version of the Combat were produced?

Educate me please!

Thanks in advance from San Jose California!
HJ

Does the bike have the "Compression Plate" (13.1685) under the cylinders? .028" thick, I think aluminum.

L.A.B. will probably know more, but I think that was to lower the compression of a "Combat" head equipped engine..

ILLF8ED · Jun 18, 2021

The ‘73 750 range came with either RH5 low compression or RH6 high compression head. The cycle parts were mostly identical to the ‘73 850MkI. 750 MkV was not really a combat engine by definition. I believe the high compression head was achieved by milling .020” as compare to .040” milled on the “C” combat head. No compression plate was issued by Norton.

ILLF8ED · Jun 18, 2021

Ron L said:
Interesting. My two 1973 750's both have 11/72 build dates and 22xxxx serial numbers. Both have RH6 heads, standard camshafts, black clock holders, silver barrels, 932 carbs, basketweave seats, and square taillights. Typical of what is considered the 'MkV' 750 (although apparently that designation was never used). Perhaps this was a case of using the stock on hand until it was depleted during the 'change' from MkIV to MkV.

Does your bike have a cylinder base gasket or a cylinder base spacer to lower compression? Have you had the barrels off and checked for the 'SS' stamp (Combat cam)?

Ron
In the 80s I owned 220627 built in Nov 1972. Same build as your’s with RH6 head, ‘73 basket weave seat, the tall 850 type instrument cans, stainless fenders and square taillight. It ended up with Interstate set up.

NORTSTER1974 · Jun 18, 2021

FYI the OP, HH, parts bikes out on Ebay 24/7. He probably scored something and wanted to know what he had. To part or not to part out? Better resale whole? Or should I chop it up? Just sayin'. Trying to figure out what to do with the bird in hand.

ILLF8ED · Jun 19, 2021

NORTSTER1974 said:
FYI the OP, HH, parts bikes out on Ebay 24/7. He probably scored something and wanted to know what he had. To part or not to part out? Better resale whole? Or should I chop it up? Just sayin'. Trying to figure out what to do with the bird in hand.

I think I know where you’re going with this. If you want to preserve it maybe you should make HH an offer. He’s a good guy.

SteveA · Jun 19, 2021

David B said:
Wow, thanks for posting that chart, David. I hadn't seen that before and didn't realize there were actually that many heads. Very interesting. I presume some are quite rare?

Just remember it isn't complete because the RH4 made a reappearance after '74!

Ron L · Jun 19, 2021

SteveA said:
Just remember it isn't complete because the RH4 made a reappearance after '74!

I have always found it interesting that the RH4 (1973) 850 head was after the RH5 and RH6 (1973) 750 heads when the 850 was not offered until March (later in the US).

Ron L · Jun 19, 2021

illf8ed said:
Ron
In the 80s I owned 220627 built in Nov 1972. Same build as your’s with RH6 head, ‘73 basket weave seat, the tall 850 type instrument cans, stainless fenders and square taillight. It ended up with Interstate set up.

My blue Roadster is still like it was when I bought it in 1973. Still original paint. The other one I bought in the mid-80's for $100. It was still altogether but not running and had a two-into-one exhaust and bad paint. I have rebuilt the engine with +0.20 pistons, new guides, valves, springs. New rod bearings. New transmission bearings and bushes. Vernier isolastics, new wiring harness. Lansdowne forks, Brembo 4 piston front disc, new rims and SS spokes. I just finished spraying the Interstate tank, fastback tail, and '71 style fastback sidecovers in '68 MGB British Racing Green. Next is some gold striping and lettering and then clear and a custom seat cover for the modified seat. Obviously not a restoration.

1973 750 Combat

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